1. Field of the Invention
The present invention relates to modified polytetrafluoroethylene (hereinafter referred to as "PTFE") fine powder and a process for preparing the same. More particularly, it relates to modified PTFE fine powder which has improved paste extrudability, rollability and drawability and affords a molded article having good sintering properties, dimensional stability during sintering, adhesivity, transparency, mechanical properties and the like.
2. Description of the Related Art
PTFE fine powder is used in the form of a molded article which is produced by paste extruding PTFE fine powder together with a liquid lubricant and optionally rolling the article to form an unsintered sheet which can be used as a sealing material. Alternatively, the molded article of the PTFE fine powder such as a sheet, a pipe and a rod is sintered before use.
When the unsintered paste extruded article of the PTFE fine powder or a rolled sheet is drawn, its porosity is decreased to give a porous material without substantially decreasing its cross section. Such drawn porous material is used as a sealing material in the unsintered state or as a continuous PTFE porous tough sheet in the sintered state.
Since the PTFE fine powder is processed by a specific method and used in special fields, the PTFE fine powder is required to have high productivity and rolling and drawing processability and further improved dimensional stability during sintering and good mechanical properties and transparency of the molded article after sintering. It is known that such required properties depend on the properties of the PTFE fine powder to be used as a raw material. However, the required properties include wide variety of properties from versatile molding properties to the properties of the final product. In the conventional PTFE fine powder, some of these properties are sacrificed to improve some other properties. For example, some PTFE fine powder is excellent in molding properties but poor in mechanical properties are. Insofar as the molding properties concerned, some PTFE fine powder has good paste extrudability but inferior drawability.
It is proposed to modify PTFE while maintaining its inherent non-melt processability by copolymerizing about 1% by mole or less of a fluorine-containing monomer with tetrafluoroethylene (TFE) (cf. U.S. Pat. Nos. 3,142,665 and 3,819,594 and Japanese Patent Publication Nos. 4643/1962, 38159/1975 and 26242/1981). The proposed copolymers are called as "modified PTFE" and are distinguished from melt processable TFE copolymers. By "modification", extrudability at a high reduction ratio (HRR) (Note: Reduction ratio=a ratio of a cross section of an extrusion die outlet to that of a cylinder portion of an extruder in which resin paste is charged), sintering properties particularly a sintering rate and transparency of the sintered article are more or less improved, although rollability (easiness of producing a thin and wide sheet by rolling the paste extruded article) and drawability are deteriorated.
It was known to increase a molecular weight of a homopolymer of TFE so as to impart a special DSC melt property to PTFE and thereby providing PTFE which have good drawability (cf. Japanese Patent Publication Nos. 54016/1981 and 39443/1983). However, the high molecular weight PTFE has inferior paste extrudability at HRR and thus poor productivity. Further, it affords a molded article having poor sintering property, and the sintered article has less self-fusion bonding or transparency.